Method of removing excess hot-dipped aluminum coating



March 28, 1967 G. E. SNIDER ETAL 3,311,495

METHOD OF REMOVING EXCESS HOT-DIPPED ALUMINUM COATING Filed July 25, 1963 2 Sheets-Sheet l A \\m\\\ \z W///////////////////( &\\\\\\\\\\\\ March 28, 1967 G. E. SNIDER ETAL METHOD OF REMOVING EXCESS HOT-DIPPED ALUMINUM COATING 2 Sheets-Sheet 2 Filed July 25, 1965 United States Patent 3,311,495 METHOD OF REMDVING EXCESS HOT-DlPPED ALUMINUM COATING Gerald E. Snider, Muncie, and Ernest R. Boiler, Marion,

Ind., assignors to The Boiler Development Corporation, Marion, Ind., a corporation of Indiana Filed July 25, 1963, Ser. No. 297,521 Claims. (Cl. 117102) This invention concerns improvements in hot-dipped metal coating processes. More particularly, the invention relates to improvements in applying hot-dipped aluminum coatings .to metals.

As is known, hot-dipped aluminum and aluminum alloy coatings, particularly on iron or steel, are of great potential value for a number of reasons. The aluminum coating in itself is quite corrosion resistant in many cases, for example, being superior in this respect to a zinc coating. The aluminum is electro-positive to iron in the presence of water in the same manner as zinc, affording galvanized protection. By virtue of its higher melting point and greater inertness of its compounds with iron, aluminum gives protection at higher temperatures than do other hot-dipped coatings. Basically, the aluminum coating is applied by dipping the work to be coated in a molten metal bath containing a substantial proportion of aluminum, although various modifications and improvements in the basic process are known such as, for example, the improved coating process described in US. Patent 2,957,782, dated Oct. 25, 1960.

When an article is coated by dipping it into a molten aluminum coating bath there is always the problem of removing the excess coating metal when the article is removed from the bath. With small articles this has been accomplished with some degree of satisfaction by centrifuging the coated article. With large articles or with articles of such configuration that centrifuging is impractical it is customary to allow the excess coating metal to drain off by gravity, sometimes assisting this with a blast of air, steam or other gas. Under these conditions a drip edge is obtained which is merely an accumulation of the coating metal along the trailing edge or surface of the article being coated.

To a large extent this accumulation of coating metal may be attributed to the surface tension of the molten coating metal, the surface tension of such metals being relatively high in comparison with other liquids. This problem is accentuated by the tendency of the molten metals to form oxide coatings producing an apparent surface tension even higher. The difiiculty in removal of excess coating metal varies considerably with the metals normally used as hot-dipped coating materials. It is most marked with aluminum because of the extremely tough oxide film formed on liquid aluminum exposed to air and because of the relatively low specific gravity of aluminum.

This problem of removing excess coating metal is present in the continuous coating of wire and strip metal. Methods for dealing with this problem have been suggested for such coating metals as zinc, lead and tin, although these known methods are somewhat cumbersome and unsatisfactory. For example, in the continuous hotdip zinc coating of wire the excess zinc is removed With a wiper usually resulting in a non-concentric coating on the wire.

With hot-dipped aluminum coating the problem of removing the excess aluminum from wire or strip is much more severe than with other metals. In the case of wire coating the nature of aluminum oxide makes it impractical to use wipers of the kind employed with other coating metals. Removal of the coated Wire directly from the bath without any Wiping leads to the formation of ice occasional drops or knobs of coating metal due to the very high surface tension of molten aluminum. In the case of strip the aluminum coated metal is presently removed with rolls in a reducing atmosphere. However, after passing through the rolls there is a tendency for the aluminum to dimple because of its high surface tension. This is more noticeable with pure aluminum than with some of the aluminum alloys, such as aluminum silicon, which form less tenacious oxide films.

. It is a primary object of this invention to eliminate the above-described difliculties in the hot-dipped coating of articles with aluminum or aluminum alloys.

A further object of the invention is to provide a method for removing excess aluminum coating metal from hotdipped coated articles in a simple and effective manner adapted for continuous and discontinuous metal coating operations.

Broadly stated, the improvement in hot-dipped aluminum coating processes constituting the present invention involves contacting the aluminum coating on the coated article while it is still in liquid condition with a second body of liquid aluminum maintained or supported on a surface separate from the article being coated. The coated article substantially immediately after dipping into the molten aluminum is brought into contact with liquid aluminum supported on an external separate surface thereby permitting the excess liquid metal on the article being coated to drain over the second surface. In the case of discrete articles the excess aluminum coating metal generally accumulates along the bottom edge usually termed the drip edge and is quickly and completely removed therefrom by contacting the coated article with the second body of molten aluminum at a point near the drip edge. In the case of continuous aluminum coating operations the excess aluminum on the article being coated is readily removed and the thickness of the final coating is maintained substantially uniform by continuously contacting the coated article in accordance with the invention.

Specific modes of carrying out the present invention are illustrated in the accompanying drawings in which:

FIGURE 1 is a fragmental sectional view illustrating an article coated with molten aluminum after removal from a hot-dip coating bath and showing the drip edge usually formed on such articles.

FIG. 2 is a similar view showing the coated article prior to contacting it with a second body of molten aluminum in accordance with the invention.

FIG. 3 shows the situation after contacting the coated article in accordance with the invention.

FIG. 4 shows the removal of excess coating metal in a continuous operation in accordance with the invention.

FIG. 5 is a perspective view showing another mode of removing excess coating metal in a substantially continuous operation in accordance with the invention.

With reference to the drawings, FIG. 1 illustrates the usual condition of an article 11 coated with a molten aluminum coating 12 after removal from the coating bath and prior to solidification or freezing of the aluminum coating. The procedure employed to coat the article can be widely varied and can, for example, be that described in the above-mentioned U.S. Patent 2,957,782. As seen, the excess aluminum coating metal collects at the trailing edge or drip edge 13 of the coated article 11. In accordance with the present invention the coated article 11 is brought into contact with a second body 14 of molten aluminum maintained on a steel or other surface 15. As shown in FIG. 3, by virtue of this procedure substantially all of the excess aluminum coating material is removed from article 11 by-fiowing over the surface 15. At this time contact between the two bodies of molten aluminum can be discontinued and the molten aluminum on article 11 permitted to freeze and form the final coating thereon. The excess aluminum collecting at the trailing edge 13 of the work piece 11 is completely removed in a matter of seconds.

In carrying out the invention it is, of course, necessary to maintain the aluminum coating metal at a sufiiciently high temperature so that it will not freeze on the coated article within the time required for substantially complete drainage of the excess metal therefrom, generally a matter of a few seconds.

In FIG. 4 strip metal 16 is coated with aluminum by dipping it into a molten bath 17 of aluminum metal. As the coated strip emerges from the bath 17, it is brought into contact with a body 18 of molten aluminum maintained on the surfaces of carbon steel sheets 19 and 20. The excess aluminum clinging to the coated wire 16 is continuously removed and drains back into the bath along the surface of the steel sheets 19 and 20, the flow path being shown by arrows. In this manner a smooth uniform aluminum coating is applied to the wire in a continuous manner,

In FIG. 5, carbon steel wire 21 is coated with aluminum by passing it through a molten bath 22 of aluminum metal. In passing through the bath 22, the wire 21 is maintained within groove 23 On rotatable sheave 24 which is immersed in the molten aluminum to a distance less than the radius thereof. Groove 23 carries a coating of molten aluminum by virtue of the sheave 24 continuously rotating within the molten aluminum bath 22. Accordingly, as the wire leaves the sheave surface and particularly the surface of groove 23, excess aluminum is removed therefrom and drains back into the bath 22.

The thickness of the coating applied to the wire depends upon the temperature of the molten aluminum bath; thicker coatings being obtained at lower temperatures and thinner coatings at higher temperatures. The configuration of the surface on which the second body of molten aluminum is maintained is not important in accomplishing the purpose of the invention. For example, in FIG. 4 the draining surfaces 19 and 20 could be flat surfaces aligned in a direction parallel to that in which the strip 16 is moving. In carrying out the invention it is preferable to avoid actual contact between the article being coated and the draining surface as this may result in a scraping action thereby preventing deposition on the wire of a uniform aluminum coating.

As previously noted, the problem of removing excess coating metal and securing a coating of uniform thickness is considerably more important with aluminum than with any of the other metals used for hot-dipped coatings because of the higher surface tension of the molten aluminum and its low specific gravity. The present invention accomplishes the removal of excess amounts of aluminum coating materials from coated articles irre spective of the size or shape thereof and insures the deposition of a smooth, uniform aluminum coating on the articles.

Those modifications and equivalents which fall within the spirit of the invention and the scope of the appended claims are to be considered part of the invention.

We claim:

1. In a process for coating a metal article with aluminum metal which comprises immersing the article in a molten aluminum coating bath and withdrawing the coated article, the improvement which comprises contacting the aluminum coating on the coated article while the aluminum coating is in liquid state with a body of liquid aluminum supported on a surface separate from the coated article and partially immersed in the aluminum coating bath to thereby allow excess aluminum coating metal to drain from the coated article to the molten aluminum coating bath.

2. A process for coating metal wire with aluminum which comprises immersing the Wire in a molten aluminum coating bath, withdrawing the wire from the molten aluminum bath, contacting the aluminum coating on said wire while the aluminum coating thereon is in liquid state with a body of liquid aluminum supported on a surface separate from the coated wire and partially immersed in said molten aluminum coating bath to thereby allow excess aluminum coating metal to drain from the coated article to the molten aluminum coating bath.

3. A continuous process for coating wire with aluminum which comprises continuously immersing wire in a molten aluminum coating bath, Withdrawing the wire from the molten aluminum coating bath, continuously contacting the aluminum coating on said wire while the aluminum coating thereon is in liquid state with a body of liquid aluminum supported on a surface separate from the coated wire and partially immersed in said molten aluminum coating bath to thereby allow excess aluminum coating metal to drain from the coated article to the molten aluminum coating bath.

4. A process for coating a strip metal article with aluminum which comprises immersing the metal strip article in a molten aluminum coating bath, withdrawing the metal strip article from the molten aluminum bath, contacting the aluminum coating on said metal strip article while the aluminum coating is in a liquid state with a body of liquid aluminum supported on a surface separate from the coated article and partially immersed in said coating bath to thereby allow excess aluminum coating metal to drain from the coated article to the molten aluminum coating bath.

5. A continuous process for coating a strip metal article with aluminum which comprises continuously immersing the metal strip article in a molten aluminum coating bath, withdrawing the metal strip article from the molten aluminum coating bath, continuously contacting the aluminum coating on said metal strip article While the aluminum coating is in liquid state with a body of liquid aluminum supported on a surface separate from the coated article and partially immersed in said molten aluminum coating bath to thereby allow excess aluminum coating metal to drain from the coated article to the molten aluminum coating bath.

References Cited by the Examiner UNITED STATES PATENTS 2,914,423 11/1959 Knapp ll7l02 ALFRED L. LEAVITT, Primary Examiner.

RALPH S. KENDALL, Examiner. 

1. IN A PROCESS FOR COATING A METAL ARTICLE WITH ALUMINUM METAL WHICH COMPRISES IMMERSING THE ARTICLE IN A MOLTEN ALUMINUM COPATING BATH AND WITHDRAWING THE COATED ARTICLE, THE IMPROVEMENT WHICH COMPRISES CONTACTING THE ALUMINUM COATING ON THE COATED ARTICLE WHILE THE ALUMINUM COATING IS IN LIQUID STATE WITH A BODY OF LIQUID ALUMINUM SUPPORTED ON A SURFACE SEPARATE FROM THE COATED ARTICLE AND PARTIALLY IMMERSED IN THE ALUMINUM COATING BATH TO THEREBY ALLOW EXCESS ALUMINUM COATING METAL TO DRAIN FROM THE COATED ARTICLE TO THE MOLTEN ALUMINUM COATING BATH. 